Cavity-enhanced spectroscopic methods have redefined the sensitivity limits of direct absorption spectroscopy. The most well-known of these techniques, cavity ringdown spectroscopy (CRDS), offers direct absorption measurements with laboratory sensitivities as low as 8.8 × 10-12 cm-1/Hz0.5 [Spence et al., 2000]. However, most of these highly sensitive instruments based on cavity-enhanced spectroscopy require actively locked cavities, high degrees of vibrational isolation, and extremely fast detection electronics [Ma et al., 1999; Spence et al., 2000]. Such systems would not be sufficiently robust for reliable laboratory use.
We have developed a host of methods to make these spectrometers more robust without significantly sacrificing sensitivity. Using integrated cavity output spectroscopy (ICOS), we can eliminate the need for fast detection electronics, and by aligning the system off-axis, we can significantly reduce the mode structure within the cavity, eliminating the need for an actively locked cavity [Paul et al., 2001; Engeln et al., 1998]. This non-resonant scheme offers very high sensitivities with a very simple system. In addition, the duty cycle of the system is much higher, and the demands on the detector are less stringent.
Projects are also underway to investigate the feasibility of cavity attenuated phase-shift (CAPS) spectroscopy [Herbelin et al., 1980; Engeln et al., 1996]. This method relies on a lock-in amplifier to measure the phase shift caused by the cavity. This phase shift is exactly analogous to the time constant measured in CRDS. However, the narrow bandwidth of this technique allows for more noise isolation and creates a simple way to monitor the reflectivity of the cavity more directly than ICOS but more easily than CRDS.
We are also investigating improvements to simple CRDS and have discovered methods to significantly improve data analysis techniques, involving statistical weighting functions that compensate for the inherent nonlinearities of the exponential transients created by the spectrometer. These improved methods promise to drastically increase the feasibility and range of systems with which the cavity-enhanced absorption techniques can be used.